An exhaust gas purification system in which a diesel particulate defuser (DPD) and a selective catalytic reduction (SCR) apparatus are connected to an exhaust pipe has been developed as an exhaust gas purification system for purifying and discharging exhaust gas from a diesel engine.
In this exhaust gas purification system, the DPD traps PM contained in exhaust gas. Further, in the exhaust gas purification system, a SCR system provided with the SCR apparatus supplies a urea solution which is stored in a urea tank to an upstream side of exhaust gas in the SCR to thereby produce ammonia by heat of the exhaust gas. Then, NOx is reduced by the thus produced ammonia over a SCR catalyst and thereby purified (see Patent Documents 1 and 2, for example).
Since the PM trapped by the DPD causes filter clogging, it is necessary to appropriately oxidize and remove the PM which has been trapped and accumulated in the DPD to thereby regenerate the DPD.
The filter clogging is detected by detecting a differential pressure between front and back of the DPD by an exhaust pressure sensor. When the detected differential pressure reaches an upper limit, DPD regeneration is automatically started by an engine control unit (ECU), or manually started in such a manner that a driver pushes a switch for performing the regeneration when a DPD warning lamp which is provided inside a cabin is lightened.
The DPD includes a diesel oxidation catalyst (DOC) which comprises an active catalyst for oxidizing unburned fuel and a catalyzed soot filter (CSF) for trapping PM contained in exhaust gas. When the DPD regeneration is performed, multiple injection (pilot injection, pre injection, main injection, and after injection) of fuel is carried out to thereby raise an exhaust gas temperature to or above a catalyst activation temperature of the DOC. After that, post injection is additionally carried out to thereby raise the exhaust gas temperature to approximately 500° C. to 600° C. The PM trapped in the CSF is then burned by this high-temperature exhaust gas, so that the PM is removed to thereby regenerate the DPD.
The DPD regeneration is automatically performed while a vehicle is running, or manually performed while a vehicle is idling after stopping. Usually, the DPD is automatically regenerated while the vehicle is running. However, since fuel oil is mixed into lubricating oil in a cylinder due to the post injection and dilution of the lubricating oil therefore occurs, the amount of the dilution is adapted to be reduced by the manual regeneration.
Further, when the vehicle is stopped during the automatic regeneration during running (hereinafter, referred to as running mode automatic regeneration), an exhaust brake valve is closed to prevent the exhaust gas temperature from decreasing so that the regeneration can also be performed during idling, thereby continuing the regeneration.
In this automatic regeneration, the temperature of exhaust gas flowing into the CSF is detected by an exhaust gas temperature sensor which is provided at a downstream side of the DOC, a deviation between the detected exhaust gas temperature and a target temperature for regeneration (hereinafter, referred to as a target regeneration temperature) (500° C. and 600° C., for example) is calculated, and a quantity of the post injection is PID-controlled based on the calculated deviation so that the exhaust gas temperature becomes the target regeneration temperature.